This invention relates to cables and in particular, but not exclusively, to telecommunications cables, which may or may not incorporate optical fibres, and methods of manufacturing them.
During use of submarine telecommunications cables hydrogen gas has been found to accumulate in the cable core. This has proved to be the case for both conventional coaxial metallic conductor cables and for optical fibre cables. The existence of free hydrogen in silica optical fibre cables is particularly undesirable since if such hydrogen can permeate into the silica of the fibres their attenuation is greatly increased, and cable performance is adversely affected, this being particularly relevant to the 1.3 and 1.55 micrometer systems.
The mechanism serving to produce the hydrogen in submarine cables is presently not fully understood and thus the present invention has, in part, proceeded from the need to prevent any hydrogen so generated reaching and adversely affecting optical fibre cores, rather than the more fundamental problem of preventing the generation of the hydrogen in the first place. The problem possibly stems from permeation of sea water into the cable construction, aided by the high pressure pertaining on the sea bed, despite the cable construction being formed is a supposedly hermetically sealed manner. Once present inside the cable the water serves to permit the build up of an electrical voltage between metallic elements therein, such as steel reinforcing wires and an aluminum tube surrounding the fibres, and this voltage serves to generate hydrogen from the sea water. (K. Mochizuki et al "Transmission loss increase in optical fibres due to hydrogen permeation" Electronics Letters 1st September 1983 Vol 18 No 18 pp 743-5).